This invention relates to the synchronization of packetized radio signals transmitted from a source of packets to a radio base station.
In cellular mobile communication systems, a single base station normally exchanges signals with a particular mobile station. As the mobile station moves, a different base station performs the signal transmission and reception function for the mobile when a mobile is xe2x80x9chanded offxe2x80x9d to another base station. This arrangement is used in analog radio transmission systems and digital systems, but is being changed where CDMA (Code Division Multiple Access) radio links are used. The characteristics of CDMA makes it possible for several base stations to transmit signals to, and receive signals from, a particular mobile station, a situation called a soft handoff; which provides a smooth transition, usually to a configuration in which only a single base station communicates with the mobile station. When the signals are received by several base stations, a frame selector unit is used to select the best signal that is received from the mobile station. If the mobile station is moving through hilly territory, or territory having many tall buildings, the strongest signal frequently moves from one base station to another. In the mobile station, provided that a digital frame is received from each of several base stations within some maximum limit, the mobile station also performs the function of selecting the strongest signal, and using that signal.
A problem arises whenever an additional base station is added to serve a call to a particular mobile station, since the latter base station must be synchronized to the other base stations already serving the call. A number of tools are available to help provide the synchronization, but these tools have not been effectively integrated into a working scheme.
The tools are the following:
1. Each base station receives an exact time from a group of satellite units used for navigational purposes. The signal from these satellites can readily be detected using the antennas of the base station.
2. The mobile switching center sends a sequence number with each frame transmitted to the base station. The frames are transmitted at 20 millisecond intervals, and arrive at the base station at substantially staggered times; the stagger in the times is greater than is tolerable by the mobile station.
The above problem is solved, and an advance over the prior art is made in accordance with this invention wherein in response to receiving a packet with a particular sequence number, the base station calculates the required time of arrival for a packet with that sequence number in order that packet can be transmitted at the time specified for that sequence number by the time derived from the satellite inputs; using that calculated time, and the actual received time, base station calculates an offset, and transmits to the mobile switching center, or other source of packets, an indication of that offset; in response to receipt of the indication, the mobile switching center arranges to advance or delay the transmission of packets by the offset. If necessary, packets to other base stations may have to be delayed by one or more, 20 millisecond interval. Advantageously, using this arrangement, all the base stations will transmit the same CDMA frame essentially simultaneously, even if the time to propagate the signals from the mobile switching center to the base stations is substantially different.